1. Field of the Invention
The present invention relates to DC-to-DC converter employing a switching circuit.
2. Description of the Related Art
FIG. 11 is a circuit diagram of a conventional isolated-type DC-to-DC converter. For example, this type of DC-to-DC converter is disclosed in “Green Electronics No. 1, Designing of Highly Efficient, Low-Noise Power Supply Circuit” published by CQ Publishing Co., Ltd. on Apr. 1, 2010 (hereinafter referred to as Non-patent Document 1).
This DC-to-DC converter is a device capable of converting DC power to AC power, transforming the AC power with a transformer, and converting the transformed AC power to DC power with a rectifier circuit for output. In the conventional device shown in FIG. 11, a full bridge circuit is employed for DC to AC conversion. In the full bridge circuit, switching elements Sa and Sb are connected in series, and switching elements Sc and Sd are connected in series. The switching elements Sa and Sb operate in a pair, and the switching elements Sc and Sd operate in a pair; two pairs of the switching elements alternately turn on or off.
In this case, as disclosed in FIG. 3 on page 67 of Non-patent Document 1, for example, if the switching elements Sa and Sd have the same turn-on time Ton and are in phase with each other and the switching elements Sb and Sc also have the same turn-on time Ton and are in phase with each other, the output voltage can be controlled by a ratio of the turn-on time Ton to the switching cycle T. However, if switching is thus controlled, hard switching occurs at on/off action of each switching element, resulting in large switching loss and decreasing conversion efficiency.
In an example disclosed in FIG. 6 on page 67 of Non-patent Document 1, therefore, so-called phase-shift control is performed by shifting the turn-on phase between the switching elements Sa and Sd and shifting the turn-on phase between the switching elements Sb and Sc, thereby enabling zero voltage switching (ZVS), i.e., enabling the switch to be turned on as the voltage across each switching element becomes zero, based on the resonance between the parasitic capacitances (capacitances between both ends) Ca, Cb, Cc and Cd of the individual switching elements and the leakage inductance LIk1 of the transformer's primary winding. With this control, the current and voltage cross time at the switching edge can be decreased to reduce the switching loss.
At this time, ZVS or so-called soft-switching can be realized such that when the switching elements Sa and Sd are kept turned on and the switching element Sd is then turned off while the switching element Sa remains turned on, the leakage inductance LIk1 tends to hold current, and the current held by this inductor charges or discharges the parasitic capacitances Cd and. Cc, whereby the voltage Vd across the switching element Sd cannot rise to the input voltage Vin rapidly, while the switching element Sc is turned on after the voltage Vc across the switching element Sc drops from Vin to 0V with a delay. As described above, ZVS is attributed to the resonance between the parasitic capacitance between both ends of the switching element and the leakage inductance LIk1, i.e., the exchange of energy stored in the inductor with the capacitance.
At light load during which a small amount of power is supplied to the secondary side, therefore, since only a small amount of current flows through the circuit, the phase-shifted ZVS full bridge circuit needs a primary resonance inductor having a large inductance. At heavy load during which a large amount of power is supplied to the secondary side, on the other hand, since a large amount of current flows through the circuit, the primary resonance inductance may be small. This is because, since the energy stored in the inductor for ZVS has a large current, even a small inductance can resonate with the parasitic capacitance between both ends of the switching element. In this case, rather, as the inductance decreases, the resonance delay time can be shortened to reduce the waiting time before transmitting the power to the secondary side. The resonance delay time is called “commutation overlap period” which reduces the maximum output voltage of the power supply.
In a DC-to-DC converter disclosed in Japanese Unexamined Patent Application Publication No. 2004-260928, therefore, an external inductance is connected in series with the leakage inductance of the transformer's primary winding so as to solve the above-mentioned problem. An external switch is connected in parallel with the external inductance so that the external switch can be turned on or off by a switching signal generator provided in a control circuit.
During light load operation, the external switch is turned off to have a large inductance that is the sum of the leakage inductance and the external inductance, realizing ZVS; during heavy load operation, the external switch is turned on to have a small inductance that is only of the leakage inductance, preventing the reduction of the maximum output voltage.
In order to turn on or off the external switch, however, the DC-to-DC converter disclosed in Japanese Unexamined Patent Application Publication No. 2004-260928 needs a detector which can distinguish between heavy load operation and light load operation by detecting the current flowing through the primary winding. Moreover, since the external switch may frequently turn on and off at the boundary between heavy load operation and light load operation, an additional countermeasure is also needed. Therefore, the circuit configuration becomes very complicated.